In the case of imbedding an anchor bolt with synthetic resin adhesive in a base material such as a concrete bed or a rock-bed, a hole having a predetermined diameter is bored to a predetermined depth in the base material with a motor-driven drill. Then, a synthetic resin adhesive, or a so-called "resin capsule" obtained by filling a container, for example a glass tube, with resin, a curing accelerator, and aggregate is inserted into the hole, and thereafter the anchor bolt is driven into the hole by being rotated and hammered by a hammer drill or a rock drill and is fixedly secured therein when the synthetic resin is cured.
In this case, the hole drilled is straight and the diameter of the hole is greater than that of the anchor bolt. For instance, for an anchor bolt 16 mm in diameter, the diameter of a hole to be drilled is 19 mm. Therefore, in the case where the adhesion of the resin adhesive to the wall of the hole is incomplete because of the undesirable variations caused in the anchor bolt imbedding work, or in the case where the base material is somewhat brittle, a predetermined pull-out strength cannot be obtained. In this case, the anchor bolt may be pulled out by the weight or vibration of a structure connected thereto, which may lead to hazards. This can be prevented by expanding the lower portion or bottom of the hole and providing there an expanded block obtained by adhering the anchor bolt to the wall of the hole, because even if the adhesion of the synthetic resin to the wall of the hole is somewhat inadequate or even if the base material is somewhat brittle, a distributed load is applied to the base material such as concrete or rock-bed when a load is applied to the anchor bolt, and therefore the predetermined pull-out strength is not decreased at all. In order to carry out such a method of imbedding anchor bolts as described above, it is necessary to provide a drilling tool with which a hole having its lower end portion expanded and tapered can be readily and positively drilled.
Accordingly, an object of the invention is to provide a drilling tool with which a tapered hole such as described above can be readily and positively drilled.